Add wxDisplay::GetStdPPIValue() and GetStdPPI()

Provide public way to access the default platform DPI, which was
previously a constant private to src/common/wincmn.cpp.

interface/wx/display.h

@@ -143,6 +143,33 @@ public:
      */
     wxSize GetPPI() const;
 
+    /**
+        Returns default display resolution for the current platform in pixels
+        per inch.
+
+        This function mostly used internally, use GetPPI() to get the actual
+        display resolution.
+
+        Currently the standard PPI is the same in both horizontal and vertical
+        directions on all platforms and its value is 96 everywhere except under
+        Apple devices (those running macOS, iOS, watchOS etc), where it is 72.
+
+        @see GetStdPPI()
+
+        @since 3.1.5
+     */
+    static int GetStdPPIValue();
+
+    /**
+        Returns default display resolution for the current platform as wxSize.
+
+        This function is equivalent to constructing wxSize object with both
+        components set to GetStdPPIValue().
+
+        @since 3.1.5
+     */
+    static wxSize GetStdPPI();
+
     /**
         Returns @true if the display is the primary display. The primary
         display is the one whose index is 0.